V-belt drives have long been used. They drive the fans and alternators on our cars. They consist of two conical pulleys and the V-belt.
If each pulley is replaced by a pair of conical sheaves whose slopes match that of the belt--we have a continuously variable friction drive. Moving one pair of sheaves toward each other causes the belt to ride to a higher radius on that pair. Moving the other pair away from each other permits the belt to fall to a lower radius on that pair. Since the ratio of speed of the driven shaft to speed of the driving shaft is determined by the ratio of these two radii, the arrangement results in a continuously variable speed drive. It is commonly called a friction drive since it is entirely dependent on the friction between the sides of the V-belt and the sheaves.
When higher power must be transmitted, the V-belt is usually replaced by a chain composed of pins extending laterally across the chain and links extending between adjacent pins. The pins are sloped to match the slope of the sheaves.
One shortcoming of this arrangement--it generates excessive noise. Each pin strikes the sheaves a hammer blow when it comes in contact. Each pin strikes the sheaves an equivalent hammer blow when it breaks loose from the sheave. Resultant vibrations pass through the system until they reach a sounding board. There they generate objectionable acoustical noise.
A second shortcoming arises from the fact that when load is applied to the tight side of the chain it is applied at the ends of the pins instead of evenly over the length of the pins. The sheaves at the first pulley pull the ends of the pin in one direction. The links pull the center of the pin in the other. Consequently the pin bows. At the second pulley the bowing is in the opposite direction. Consequently the load is born by the outer rows of links. The middle rows may carry no load at all or may even be subjected to compression. For any given chain load a chain one be designed for which the distance between holes in the outer links is sufficiently greater than the distance between holes in the inner links that the load is evenly distributed among the links. But for either greater or lesser loads the load is again unevenly distributed. This problem of uneven distribution of load becomes particularly bothersome when vibration is considered.
This second shortcoming may be largely overcome by eliminating the center links of the chain. Place one row of links along the first edge of the chain and a second row along the second edge. However, this is not done at present.